A method and apparatus for simultaneously exposing an optical fiber 32, made of a material which has a refractive index that can be permanently changed by exposure to light of a predetermined intensity at a predetermined wavelength, to two writing light beams 268,272 which intersect and interfere in a predetermined region 30 of the fiber 32 so as to create an interference pattern along a longitudinal axis of the fiber 32, for a time long enough to form a permanent refractive index variation in the waveguide. Each of said two writing beams have a predetermined number of wavelengths, each wavelength being sufficiently different from each other so as to cause said refractive index variation to be an aperiodic variation. Also, each wavelength represents a Fourier component of the desired aperiodic refractive index variation.